What fire detection type (flame or heat) is best for transformer fire detection?
"Best" can be defined so many different ways . . .
Cheapest?
Quick-acting to disconnect / remove faulted / burning elements from the power system so as to minimize power system disturbance?
Quick-acting to disconnect / remove faulted / burning elements from the power system so as to minimize equipment damage?
Quick-acting to disconnect / remove faulted / burning elements from the power system so as to minimize / eliminate hazards / injuries / fatalities to employees / the public?
Selectivity/ reliability, i.e. for outdoor equipment will not be fooled into erroneous operation for weather-related phenomena?
Tied directly into fire suppression systems that will promptly extinguish fires and capture spills so as to minimize environmental impact?
Note that a good number of the factors mentioned in the far-from-exhaustive list above will in many cases suggest conflicting solutions. It is therefore at this point that the input of numerous specialists and stakeholders as demanded by the situation is very valuable, viz., electrical, chemical and civil engineers, risk analysis experts, your insurer, the relevant AHJs, etc., etc.
CR
"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]